Conversion of HPV 18 positive non-tumorigenic HeLa-fibroblast hybrids to invasive growth involves loss of TNF-alpha mediated repression of viral transcription and modification of the AP-1 transcription complex

AP-1 represents a transcription factor, which plays a pivotal role in initi ating and maintaining the expression of human papillomavirus (HPV) oncoprot eins E6 and E7 during HPV-linked carcinogenesis of the uterine cervix. AP-1 stands as a synonym for different proteins such as c-Jun, JunB, JunD, c-Fo s, FosB as well as the Fos-related antigens Fra-1 and Fra-2, which can eith er homo- or heterodimerize to build up a functional transcription complex, AP-1 is mainly considered as a positive regulator, which binds to cognate D NA sequences within the viral upstream regulatory region, By using nontumor igenic HeLa-fibroblast hybrids ('444'), their tumorigenic segregants ('CGL3 ') as well as HPV 18 positive HeLa cells as a experimental model system, ev idence is provided that AP-1 composition differs considerably between these cell lines, In nuclear extracts obtained from non-tumorigenic cells, Jun-f amily members (in the order c-Jun > JunD > JunB) were mainly heterodimerize d with Fra-1, a protein, known to be involved in the abrogation of AP-1 act ivity under certain experimental conditions, In contrast, Fra-1 concentrati on is low in extracts from tumorigenic cells, Conversely, c-Fos, the canoni cal dimerization partner of Jun proteins is expressed in substantial quanti ty in HeLa- and 'CGL3' cells, but it is completely absent in AP-1 complexes from non-tumorigenic '444' cells, Ectopical expression of c-fos under a he terologous promoter in '444'-cells induces tumorigenicity and a change of t he Jun/Fra-1 ratio towards a constellation initially detected in 'CGL3'-and HeLa cells. Furthermore, conversion to tumorigenicity is accompanied with a resistance against TNF-alpha, a cytokine, capable to selectively suppress HPV 18 transcription in formerly non-malignant cells, These data propose a novel role for AP-1 as an essential component of an inter- and intracellul ar surveillance mechanism negatively controlling HPV transcription in non-t umorigenic cells.